1. Field of the Invention
The present invention relates generally to methods of providing modulation signals to electrical circuits and, more particularly, to a method and apparatus for providing a generalizable pulse width modulation signal using either trailing edge or leading edge modulation.
2. Description of the Background Art
Switching converters are commonly used to provide DC to DC, DC to AC, AC to DC and AC to AC conversion of electrical waveforms. These converters are often categorized by applicable circuit configuration and/or capabilities, and include buck converters (where the load voltage is less than the source voltage), boost converters (where the load voltage is greater than the source voltage), buck-boost converters (which are capable of either), Cuk converters, Watkins-Johnson converters, and A, B, and C quadratic converters. All of these converters rely on switching control and, furthermore, the performance of electrical circuits, including converters, can be improved by closed loop control of a control variable selected by the designer.
Constant frequency pulse-width modulation (PWM) is a well-known method to control switching converters. Generally speaking, PWM allows a switching converter variable to be controlled by modulating the width of a control pulse in each switching cycle. Generally, output-voltage, output-current, input-current, or any other variable or combination of variables are eligible as controlled-variables for a specific control task.
Many methods are available to modulate the pulse-width including, but not limited to, comparing a modulation reference to a constant frequency linear or non-linear carrier, by current-mode control, or by one-cycle control as described in reference 1!. For each method, PWM may be performed on trailing-edge, leading-edge, or both edges of the output-pulse. These are referred to as trailing-edge, leading-edge, and double-edge modulation, respectively. These three modulation types differ little from each other at low frequency from a dynamics standpoint. However, it has been observed that their characteristics at high frequency may be distinct, as described in references 2! and 3!.
Most commercially available PWM control chips employ trailing-edge modulation but, for some applications, leading-edge modulation is more desirable, such as the secondary side PWM control chip described in reference 4!. Further, leading-edge modulation can sometimes result in simpler control circuitry.
However, while PWM control is often relatively simple in theory, it is not in application. While modulation equations describing the required mathematical relationship between the controlled variable and the modulation equation may be expressed in closed form, implementing these mathematical relationships in electrical circuitry to effect the desired control scheme is often a complicated, time-consuming, and expensive process, requiring extensive analysis, special electrical components, or additional circuitry.
Therefore, there is a need for a pulse-width modulator which allows the system and circuit designer to implement a wide variety of constant-frequency control schemes without the need for special circuit components or specially designed circuits requiring extensive analysis and design. The present invention meets this need, as well as others, with a generalized pulse-width modulator that allows modulation expressions to be easily and directly implemented in electronic hardware.